💡 Our Technical Review in summary
Summary
Exchange Online users experienced delays in sending and receiving emails when routing traffic through a third-party email protection service. The issue originated from a misconfigured cache limit within the Simple Mail Transfer Protocol (SMTP) mail flow process. This misconfiguration caused the mail transport service to send an excessive volume of requests to downstream components, resulting in “451 4.3.2 Temporary server error” responses.
Impact
The impact was observed across all Exchange Online connection methods, including Outlook on the web, desktop clients, and mobile apps. Specifically, it affected:
- Mail Flow: Delays in both inbound and outbound email delivery.
- Scope: Organizations utilizing SMTP connection methods integrated with a specific third-party email protection service.
- Duration: The issue persisted from January 26, 2026, at 6:00 AM UTC to January 30, 2026, at 6:25 PM UTC.
Status
Service Restored. Microsoft has fully implemented a configuration change to correct the cache limit settings. Internal telemetry has confirmed that mail flow has returned to normal levels. Microsoft is continuing to research the event to prevent future occurrences.
Microsoft Official Issue Details
User Impact
Users’ Exchange Online email routed through a third-party protection service may have been delayed.
Status: serviceRestored
Service: Exchange Online
Classification: incident
Origin: microsoft
Updates
- 2026-01-30 19:26
Title: Users routing their Exchange Online email through a third-party protection service may have send and receive delaysUser impact: Users’ Exchange Online email routed through a third-party protection service may have been delayed.
More info: This issue specifically impacted users who use Simple Mail Transfer Protocol (SMTP) as their Exchange Online connection method and routed their email messages through a specific third-party email protection service. The email delays, which impacted both sending and receiving email, occurred in all Exchange Online connection methods.
Final status: We’ve fully implemented our identified configuration change and confirmed using internal telemetry that it effectively mitigated this issue. We determined that a cache limit configuration that helps facilitate the impact scenario process with SMTP mail flow was misconfigured, resulting in impact.
Scope of impact: This problem may have impacted any user sending or receiving email messages when leveraging a specific third-party email protection service.
Start time: Monday, January 26, 2026, at 6:00 AM UTC
End time: Friday, January 30, 2026, at 6:25 PM UTC
Root cause: A cache limit configuration that helps facilitate the impact scenario process with SMTP mail flow was misconfigured, resulting in impact.
Next steps:
– We’re continuing our research into identifying the root cause of this impacting event to better understand how to prevent it from happening again in the future.This is the final update for the event.
- 2026-01-30 17:49
Title: Users routing their Exchange Online email through a third-party protection service may have send and receive delaysUser impact: Users routing their Exchange Online email through a third-party protection service may have send and receive delays.
More info: This issue specifically impacts users who use Simple Mail Transfer Protocol (SMTP) as their connection method and route their email messages through a specific third-party email protection service. The email delays, which impacts both sending and receiving email, occurs in all Exchange Online connection methods.
Current status: Though our investigation into the root cause of this impacting issue is ongoing, we’ve identified and implemented a configuration change in effort to mitigate the problem. We’re reviewing our internal telemetry to assess if the configuration change was effective. Please note that we’ve updated the Title, User impact, and More info of this communication to better reflect the impact of the event.
Scope of impact: This problem may impact any user sending or receiving email messages when leveraging a specific third-party email protection service.
Next update by: Friday, January 30, 2026, at 8:00 PM UTC
- 2026-01-29 21:19
Title: Users may experience delays when sending or receiving Exchange Online email messagesUser impact: Users may experience delays when sending or receiving Exchange Online email messages.
More info: This issue specifically impacts users who are on Simple Mail Transfer Protocol (SMTP) connection methods, which include Outlook on the web, the Outlook desktop clients, and the mobile apps.
Current status: Our investigation has further identified that exceptions may be occurring as our mail transport service is sending an unexpectedly large amount of requests to downstream components in the mail routing code path. We’re investigating recent changes to confirm this theory and determine next steps for remediation.
Scope of impact: Some users attempting to send or receive Exchange Online email messages may be impacted.
Next update by: Friday, January 30, 2026, at 7:30 PM UTC
- 2026-01-29 19:19
Title: Users may experience delays when sending or receiving Exchange Online email messagesUser impact: Users may experience delays when sending or receiving Exchange Online email messages.
More info: This issue specifically impacts users who are on Simple Mail Transfer Protocol (SMTP) connection methods, which include Outlook on the web, the Outlook desktop clients, and the mobile apps.
Current status: We’ve identified the “451 4.3.2 Temporary server error” occurring that’s leading to impact. This error, generally, is a common error, however our review of transport service health telemetry confirmed an increase in the rate of this error generating. We’re investigating why we see an increase of this error resulting in impact to better understand how the issue is manifesting and prepare next steps for potential remediating actions.
Scope of impact: Some users attempting to send or receive Exchange Online email messages may be impacted.
Next update by: Thursday, January 29, 2026, at 9:30 PM UTC
- 2026-01-29 18:05
Title: Users may experience delays when sending or receiving Exchange Online email messagesUser impact: Users may experience delays when sending or receiving Exchange Online email messages.
More info: This issue specifically impacts users who are on Simple Mail Transfer Protocol (SMTP) connection methods, which include Outlook on the web, the Outlook desktop clients, and the mobile apps.
Current status: We’re reviewing transport service logs in parallel with samples of affected messages to identify and isolate any errors that may be contributing to impact for further investigation.
Scope of impact: Some users attempting to send or receive Exchange Online email messages may be impacted.
Next update by: Thursday, January 29, 2026, at 7:30 PM UTC
- 2026-01-29 17:36
Title: Users may experience delays when sending or receiving Exchange Online email messagesUser impact: Users may experience delays when sending or receiving Exchange Online email messages.
Current status: We’re investigating a potential issue with Exchange Online and checking for impact to your organization. We’ll provide an update within 30 minutes.
